The invention relates to attaching a porous layer to a dense rigid support and can be particularly advantageously applied to the making of prostheses, more particularly hip prostheses.
The majority of hip prostheses comprise an intramedullary rod sealed to the femur by an acrylic cement. As it sets, the cement causes an exothermic polymerization. A considerable amount of heat is given off and the temperature can locally reach values of teh order of 60.degree. C. This results in an appreciable primary necrosis of the surrounding tissues. Another disadvantage of this kind of prosthesis results from the fact that the doefficient of thermal expansion of the cement, which is ten times higher than that of the adjacent bone tissue, causes on cooling a gap between the bone and the cement which may be as large as several microns in certain places. The cement layer is moreover sintered on to the metal rod of the prosthesis and may become cracked. The resulting debris also sooner or later induces necrosis in the adjoining bone tissue, such necrosis finally leading to the unsealing of the rod.
A number of methods eliminating the use of cement have been suggested to obviate these disadvantages. Generally in these methods scoring or irregular roughnesses are created by milling or moulding on the tail of the femur to increase the contact surface with its medullary canal. This method avoids the use of cement, since the bone tissue directly contacts the prosthesis by rehabitation of the relief thus created, but such rehabitation is unreliable, since generally it does not take place in optimum conditions and remains difficult to control.
Other more reliable methods have been suggested which produce the porous zone by powder metallurgy.
International Pat. No. 83/00282 discloses a process wherein the porous zone is obtained by coating the surface of the prosthesis with a mixture of two different powders, for example, titanium and iron. After partial densification, one of the two powders is eliminated, for example, by dissolution, leaving considerable empty spaces between the grains of the other powder.
European Pat. No. 075378 discloses a process wherein the porous zone is obtained directly by sintering with powders of different grain sizes. A first powder is brought into contact with the surface of the prosthesis corresponding to the tail of the femur, while a second powder is brought into contact with the part of the prosthesis corresponding to the femur head. A judicious selection of the powders and control of the densification parameters enables a porous zone to be obtained on the side of the tail of the femur and a dense zone on the side of its head simultaneously.
Although these two last-mentioned processes are more reliable than the preceding ones, they still have a number of disadvantages, mainly due to the fact that in both cases the porous zone is metallurgically connected to the support by diffusion. However, two phenomena contribute towards the degradation of the mechanical properties of a hip prosthesis rehabitable by the bone tissue when the prosthesis is formed by dense metallic support (which can be formed by titanium, a titanium alloy or a cobalt- or iron-based alloy) and a porous coating adhering to the surface of the supprt. Elevated temperatures (of the order of 1150 to 1450.degree. C.) are required for producing the connection between the support and the coating, more particularly in the case of titanium alloys, which are the ones most widely used. Such temperatures lead to a considerable enlargement of the .beta. grain and the formation of an .alpha. grain joint, something which degrades the mechanical characteristics of the support. This sintering temperature can be reduced by means of sintering adjuvants (for example, copper or silver), and this causes a local displacement of the transition temperature between the .alpha./.beta. structure and the .beta. structure. This contributes towards the creation of a connecting layer of .beta.structure. The change in structure increases sensitivity to the propagation of cracks in the titanium alloys which are moreover highly sensitive to the notching effect--i.e., the start of a crack.
However, a metallurgical connection of a spherule to a titanium alloy support is equivalent to a notch machined in a smooth support. This is illustrated by the annexed FIG. 1, which shows a spherule 10 metallurgically connected to a substrate 12. It can be seen that the spherule 10 is attached to the substrate 12 via a neck 14 of very small radius. Since diffusion has taken place between the spherule and the substrate, everything takes place as if this were a single object in which a notch having the shape of the neck 14 had been produced.
The equivalent coefficient of stress intensity K.sub.t will be higher in proportion to the smallness of the connecting radius or the fritting neck between the spherule and the support.
This geometry is observed in processes using natural sintering (without external stresses) with or without sintering adjuvant. If such an adjuvant is used, the temperature of appearance of the first liquid higher than the temperature of conversion of the .alpha. structure into the .beta. structure contributes towards the formation of .alpha. grain joints on cooling in the forged support and towards a partial dissolution in the spherules forming the porous zone. The formation of small .alpha. plaquettes extending through the diffusion zone (reference 16 in FIG. 1) contributes towards the fragility of the connection under stressing. In the case of a porous layer connected to a titanium alloy support, the problem arises of the presence of notches in a structure particularly unfavorable to the strength of the support. The reduction in the mechanical characteristics of such composite elements is considerable, with a limit of endurance which may be only 20% of that measured in such alloys when the experiment is performed with smooth test pieces.
It is an object of the invention to obviate these disadvantages by providing a process for attaching a porous layer to a substrate which can be used more particularly for making prostheses and which enables the spherules to be readily attached to and satisfactorily retained on the support.